1. Field of the Invention
The present invention relates to a method of crystallizing an amorphous silicon film, and more particularly, to the crystallizing of the amorphous silicon film using an excimer laser.
2. Description of Related Art
Generally, polycrystalline silicon (p-Si) or amorphous silicon (a-Si) are materials used as the active layer of thin film transistor (TFTs) in liquid crystal display (LCD) devices. In contrast to amorphous silicon, polycrystalline silicon provides faster display response time when used as an element of the TFT. However, when using polycrystalline silicon as the active layer of the TFT, a quartz substrate is required because of high temperature processes used to form the TFT, thereby increasing costs. Accordingly, polycrystalline silicon is generally used for small-sized substrates, on the order of 3 or 4 inches. Alternatively, amorphous silicon may be formed under the relatively low temperature processes, and thus, it is used for forming the TFT of a large-sized LCD. However, an image produced on the LCD may flicker because dangling bonds of the amorphous silicon may cause light leakage current when light is irradiated onto the LCD panel. Therefore, it has been proposed that in order to use polycrystalline silicon for the large-sized LCD, amorphous silicon is melted and crystallized.
Polycrystalline silicon is composed of crystal grains and grain boundaries. If the grain boundaries are irregularly distributed within the polycrystalline silicon, the grain boundaries act as barriers to the carriers, thereby degrading carrier mobility. Accordingly, when crystallizing amorphous silicon to form polycrystalline silicon, uniformly large grains need to be formed to reduce the effects caused at the grain boundaries.
One proposed process for crystallizing amorphous silicon utilizes laser heat treatment. The laser heat treatment process involves depositing amorphous silicon onto a substrate and repeatedly irradiating the amorphous silicon with laser energy, thereby forming polycrystalline silicon with uniform grains. One common example of laser heat treatment involves excimer laser crystallizing. The excimer laser usually uses halides such as ArF, KrF or XeCl, for example. Furthermore, the energy density distribution profile of an excimer laser beam is trapezoid-shaped, unlike other laser beams which have a Gaussian energy density distribution profile resembling an isosceles triangle. The characteristic curve of the Gaussian beam is different from the characteristic curve of the excimer laser beam. By passing a laser beam through a beam homogenizer, the Gaussian energy density distribution is transformed into an energy density uniformly distributed across the spatial extent of the beam.
FIG. 1 is a schematic view of an excimer laser apparatus that crystallizes amorphous silicon, FIG. 2 shows a Gaussian energy density distribution profile of a laser beam generated from a excimer laser generator, FIG. 3 shows a segment of a laser beam where each of the segments of the laser beam cross, and FIG. 4 shows a top hat distribution profile of the energy density after the laser beam passes through a beam homogenizer.
In FIG. 1, the excimer laser apparatus includes an excimer laser generator 50 and a beam homogenizer 60. A medium for the excimer laser generator 50 is HCl, Ne or/and a mixture thereof. A laser control system (not shown) installed in the excimer laser generator 50 controls energy discharged from the excited electrons and generates a laser beam having a desired energy density. The laser generator 50 emits the laser beam to the beam homogenizer 60. Furthermore, the beam homogenizer 60 transforms a laser beam 52 into a laser beam 56 having a uniform energy density distribution across the spatial extent of the beam, thereby resulting in a top hat energy density distribution profile 58 as shown in FIG. 4.
Before the laser beam 52 passes through the beam homogenizer 60, it has a Gaussian energy density distribution as previously described and shown in FIG. 2. An energy density of the laser beam is a relative maximum within a central portion of the laser beam, thereby forming a Gaussian energy density distribution profile that resembles an isosceles triangle. As the laser beam 52 is divided in seven segments “S” by the beam homogenizer 60, each of the individual segments of the laser beam cross each other in one segment in the beam homogenizer 60. Accordingly, the laser beam 52 is transformed into the laser beam 56 having a top hat energy density distribution profile 58. The beam profile of the laser beam 56 has a trapezoidal shape and is applied to the substrate 70.
FIG. 5 shows the energy density of a laser beam having a top hat distribution profile. In FIG. 5, the x-axis represents a width of the laser beam, while the y-axis represents the energy density of the laser beam. The energy density “ET” is a top hat area of the excimer laser beam and represents an area with a sufficient amount of energy density to melt amorphous silicon. The energy density “ET” is controlled to be between a second energy density “E2” and a third energy density “E3.” Further, an arrow 80 disposed below the graph represents a direction in which the excimer laser beam is irradiated onto the surface of the amorphous silicon that is disposed on a substrate. In practice, the substrate moves in a direction opposite to the direction of the arrow 80, and the laser beam crystallizes the amorphous silicon.
FIG. 6 is a cross-sectional view of a substrate having an amorphous silicon layer that is irradiated with a laser beam having the top hat energy density distribution profile shown in FIG. 5. In FIG. 6, an insulation layer 10 is disposed on a transparent substrate 1 includes an insulator layer 10 made of silicon nitride (SiNx), for example. An amorphous silicon layer 20 is disposed on the insulation layer 10 with seeds 15 formed in a bottom region of the amorphous silicon layer 20 adjacent to an interface between the insulation layer 10 and the amorphous silicon layer 20. The seeds 15 enhance growth of the grains in the amorphous silicon and are usually formed by hydrogen (H2) gas during a PECVD (Plasma Enhanced Chemical Vapor Deposition) process that sequentially deposits the insulation layer 10 and the amorphous silicon layer 20 onto the substrate 1.
Now referring to FIGS. 5 and 6, the energy density necessary to crystallize the amorphous silicon is divided into three energy densities: a first energy density “E1;” a second energy density “E2;” and, a third energy density “E3.” The first energy density “E1” ranges from 200 to 300 mJ/cm2 and melts a surface portion of the amorphous silicon layer 20, thereby forming fine grains having diameters of about 1,000 angstroms (Å). The second energy density “E2” ranges from 310 to 370 mJ/cm2 and melts the amorphous silicon layer 20 to the bottom region around the seeds 15 to form large grains. However, since the seeds 15 are irregularly distributed, the large grains are not uniformly formed. The third energy density “E3” is at least 380 mJ/cm2 and melts all portions of the amorphous silicon layer 20, thereby forming polycrystalline silicon. During crystallizing by the third energy density “E3,” the seeds 15 are re-formed within the polycrystalline silicon layer and the number of the seeds increases, thereby forming fine grains having diameters of 500 angstroms (Å). Accordingly, it is difficult to create large grains using the above-described process since the amorphous silicon is repeatedly crystallized. Furthermore, since the laser beam irradiates only a single portion of the substrate at a time, grains are formed by repeatedly melting and crystallizing the amorphous silicon layer.
However, the laser heating treatment process described above is problematic in that the excimer laser apparatus produces large energy density differences of 15% between subsequent laser beam exposures and the resulting crystallizing of the amorphous silicon has a narrow distribution. Furthermore, in order to overcome these problems, the subsequent laser beam exposures are overlapped in an increasing ratio. Accordingly, since this process lengthens the exposure time and the total number of the exposures, the useful life of the excimer laser apparatus is shortened, thereby decreasing manufacturing yields.
In the conventional art, it is very difficult to create an ideal energy density that can uniformly form the grains due to energy density differences between sequential exposures. Accordingly, the laser irradiation exposure overlaps at an increasing ratio, and the laser irradiation exposure time increases. Further, a large number of lasers is required, thereby increasing the costs maintaining the laser apparatus. However, in the present invention, since the energy density of the laser has a stepped distribution profile and the difference of the energy density is about 10 to 15 mJ/cm2 between the first energy density “EM” and the second energy density “ES,” the laser beam may be overlapped at a low ratio. Therefore, the number of irradiation exposures and the irradiation exposure time are reduced. Further, the grains have a relatively large uniform size of about 3,000 to 4,000 angstroms (Å). In other words, the laser irradiation exposures are overlapped at the ratio of about 95% and it takes about 120 seconds to crystallize one substrate at the condition of 360 mm scan/300 Hz in the conventional art. However, in the present invention, the overlapping ratio of the laser irradiation exposure is less than 90% and the irradiation exposure time to one substrate is one sixth of the conventional time to one substrate, thereby increasing the throughput of the excimer laser apparatus.